The pumps used in hydraulic elevators are almost exclusively screw pumps. An important reason for this is that screw pumps have good power and volume transmission characteristics. Especially in elevator drives, but also in other applications, the pressure pulsations produced by the pump are a problem. In screw pumps, the pressure pulse level is fairly low. However, even this low pressure pulse level generates noise and vibration in the hydraulic circuit, requiring investments to damp these, thereby increasing the costs. If undamped, the noise and vibration have a disturbing effect at least on elevator passengers and possibly other people as well, once the noise or vibration has propagated further away from the pump via the building structures, air or hydraulic circuit. The pressure pulses also have a negative effect on the pump, hydraulic circuit and other equipment to which the pressure pulses or the vibrations they produce are conducted.
In a screw pump, pressure pulsation is caused by two significant factors, viz. compressibility of the oil and variation of leakage flow in the pump. The variation in leakage flow depends on the variation in the tightness of the pump during the pumping cycle; in other words, the number of chambers formed between the pump screws and therefore also the total number of sealings between chambers varies while the screws are being rotated. Thus, high pressure conditions occur at intervals. On the other hand, compressibility results in pressure pulsation when the space between the pump screws opens at the pressure end of the pump and the pressure difference is suddenly levelled out, leading to a momentary drop in the pressure delivered by the pump. In order to eliminate the pressure pulsation or at least to reduce it to a level where it would be insignificant enough to allow it to be ignored in the design of the hydraulic circuit or other constructions, e.g. the structures of a hydraulic elevator, it would be necessary to solve both the pressure pulsation problem resulting from compressibility of oil and the pressure pulsation problem resulting from leakage flow. Previously known screw pump solutions, however, do not eliminate pressure pulsation completely or even nearly completely.
From German patent specification no. 4107315, a screw pump is known which has a driving screw and at least one side screw. Both the driving screw and the side screw are placed in the casing enclosing the screws between a pressure space and a suction space. The screw end on the pressure side is tapered. The screw tapers by a factor of max. 0.4 over a distance corresponding to the screw pitch. The tapering angle is below 10.degree.. The tapering is designed to achieve gradual and defined opening of the pressure-side chamber. In this way, the pressure pulsation and the resulting pulsation of the flow are clearly reduced, but still a pressure pulsation of significant magnitude remains.